1. Field of the Invention
This invention relates in general to a read/write elements, and more particularly to method, apparatus and program storage device for providing protrusion feedback for a read/write element.
2. Description of Related Art
Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. In such devices, read-write heads are used to write data on or read data from an adjacently rotating hard or flexible disk.
Existing magnetic storage systems use magnetoresistive (MR) heads to read data from magnetic media and to write data onto magnetic media. MR disk drives use a rotatable disk with concentric data tracks containing the user data, a read/write head that may include an inductive write head and an MR read head for writing and reading data on the various tracks, a data readback and detection channel coupled to the MR head for processing the data magnetically recorded on the disk, an actuator connected to a carrier for the head for moving the head to the desired data track and maintaining it over the track centerline during read or write operations.
There is typically a plurality of disks stacked on a hub that is rotated by a disk drive spindle motor. A housing supports the drive motor and head actuator and surrounds the head and disk to provide a substantially sealed environment for the head-disk interface. The head carrier is typically an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained in very close proximity to the disk surface by a relatively fragile suspension that connects the slider to the actuator. The spacing between the slider and the disk surface is called the flying height and its precise value is critical to the proper function of the reading and writing process.
The inductive write head and MR read head are patterned on the trailing end of the slider, which is the portion of the slider that flies closest to the disk surface. The slider is either biased toward the disk surface by a small spring force from the suspension, or is “self-loaded” to the disk surface by means of a “negative-pressure” air-bearing surface on the slider.
The MR sensor detects magnetic field signals through the resistance changes of a magnetoresistive element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in an MR read head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage. The conventional MR sensor used in magnetic recording systems operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is “pinned”, and thus prevented from rotating in the presence of an external magnetic field. This type of MR sensor is called a “spin valve” sensor.
The read-write heads have been designed so that they will fly over the surface of the rotating disk at a very small, though theoretically constant distance above the disk. The separation between the read-write head and the disk is called the flying height, and is maintained by a film of air. The flying height is critical to proper function during reading and writing. If the flying height is too high during read, the read head will not be able to resolve the fine detail of the magnetic signal, thereby resulting in undecipherable data. Similarly, if the flying height is too high during a write, the magnetic flux lines that intersect the plane of the disk surface become weaker, thereby leading to loss of resolution.
As magnetic recording areal density increases, the fly height between the head and the disk continues to shrink. As discrete data storage areas are placed more closely to one another, the transducer must be positioned more closely to the recording surface to distinguish between adjacent storage areas. In recent year, transducing head flying heights have been decreased largely due to improved techniques for reducing media surface roughness. Further reductions in flying height are enabled by a super smooth polishing of media surfaces in data recording areas while also providing an adjacent head contact zone, textured to avoid stiction problems.
There are several factors that limit the reduction in slider flying height. These factors might reasonably be ignored at earlier flying heights, but would become major concerns at today's target flying heights. Factors that limit the reduction in slider flying height include variations in the sliders themselves, variations in the structure that supports the sliders, and media surface roughness.
More particularly, normal tolerances in slider fabrication lead to structural variations among the sliders in any given batch. Consequently, the flying heights of sliders in a batch are distributed over a range, although the flying height of each slider individually is substantially constant.
Disk roughness is also a problem at lower slider flying heights because maximum peaks are more likely to protrude into a normal range of slider operation. Thus, the probability of unintended and damaging slider/disk contact increases. The risk of damage from these discontinuities is greater at lower slider flying heights.
Minute slider flying heights also exaggerates thermal effects. Thermal effects include the natural tendency of materials to expand when heated, quantified by a temperature coefficient of thermal expansion more conveniently called a thermal expansion coefficient. Materials with higher coefficients expand more in response to a given temperature increase. When materials having different thermal expansion coefficients are contiguous and integral, their differing expansion when heated leads to elastic deformations and elastic restoring forces in both of the materials. Reduced flying heights increase the need to take thermal expansion and thermally induced elastic deformation into account.
When the magnetic head is operating within a disk drive its operating temperature may reach very high levels. These high temperatures are at least partly induced by the write current heating of the coil and yoke during recording. Other factors contributing to the heating include the disk velocity, contact with asperities, the frequency of the write bursts, etc. These high temperatures cause the hard baked photoresist insulation stack to expand more than the overcoat layer, which causes the overcoat layer to protrude beyond the pole tips at the air-bearing surface (ABS). This protrusion can ruin the head or severely degrade its performance. Further, the hard baking, of the photoresist layers can result in loss of signal amplitude for some read sensors, such as spin valve sensors, in an adjoining read head. The hard baked temperatures cause some intermixing of the materials of the layers, which can significantly degrade their performance. Still further, the hard baked photoresist insulation stack has poor heat dissipation, which aggravates all of the aforementioned problems.
Sliders with heating elements to control the level of read/write element protrusion are being developed. One aspect of the design of these sliders is to force protrusion of the read head so that the read head and the write head are at the same level. However, it is difficult to determine the level of protrusion, i.e., the height of the protrusion, because there is no feedback system.
It can be seen then that there is a need for a method, apparatus and program storage device for providing protrusion feedback for a read/write element.